Electrosurgical devices or systems have been in use for a considerable time. For example, EP 1 053 720 A1 discloses an electrosurgical system, incorporating a surgical instrument and a device, which provides the electrosurgical instrument with HF power. An HF generator, which delivers the corresponding HF voltage, is used for this purpose. The generator is switched on and off in rapid sequence to regulate the power converted at the electrode, where power regulation is possible by setting the duty cycle. This is intended in particular to prevent overheating of the electrode.
To determine the electrode temperature, it is proposed that the voltage at the electrode be investigated for DC components so that a thermionic effect, i.e. a temperature dependent electron emission, can be detected if the electrode becomes too hot. The connected processor establishes this and regulates the duty cycle of the HF voltage to bring about the desired reduction in power. The intention here is to avoid damage to the electrode.
It is further known, for example from US pre-grant publication 2009/0209956 A1, how to monitor the crest factor of the electrode current for an electrosurgical device to draw inferences about whether the electrode is in an incision mode or in a cutting mode. In the incision mode, the electrode is surrounded by water that can conduct ions, e.g., an NaCl solution. It carries a relatively high current. This leads to the development of a vapor bubble at the electrode, in which a plasma discharge can then develop. Once this happens, the crest factor of the flowing current changes significantly. The change occurring in the crest factor is recognized and evaluated for the further control of the device.
It has been found that in particular the transition from the incision to the cutting/ablation phase can be critical. The water enveloping the electrode evaporates very rapidly, so that the power required and actually supplied to the electrode in the incision phase has to be reduced rapidly. If this does not happen, then it is possible for too much power to be supplied in the commencing plasma discharge, leading to undesired effects. On the other hand, it is necessary to work with high power in the incision phase to bring about a reliable development of a vapor bubble and plasma discharge.